Mariko was a teenager the day she found herself alone in a near-empty carriage with a man who sat across from her, exposed himself and began to masturbate. Terrified that fleeing or crying out might provoke something worse, she fixed her gaze elsewhere and waited for the next station. “There was nothing I could do,” said Mariko, now 33, who asked that her family name not be published. “I was terrified that he might attack me, so I kept quiet.” The memory has never entirely left her. It still...